A microstrip antenna element is fabricated using conventional printed circuit board manufacturing and photoetching techniques. A dielectric sheet or substrate is clad on one side with a metal, such as copper or a similar conductor. The metal layer is etched to a predetermined pattern to form a radiator or antenna element. The pattern is typically either round or square, depending on the application. The metal on the other side of the dielectric sheet forms a ground plane. Generally, coaxial feed probes are fed from the ground plane side, through the substrate and to metal antenna element for coupling the antenna element to an external circuit. However, line feeds may also be formed on the surface of the dielectric sheet to couple the antenna element to a circuit. Once the metal is etched to form the antenna element, a radome comprised of a dielectric is bonded to the dielectric substrate, over the metal layer. A multi-layer microstrip antenna is constructed by bonding together two or more antenna elements, each typically separated from the other by an additional sheet of dielectric substrate.
The resonant frequency of a microstrip antenna element depends on the thickness of the dielectric substrate and its dielectric constant. Typically, in order to obtain wide hemispheric coverage with a small antenna size, a substrate with a comparatively high dielectric, typically greater than 6, is chosen. Vendors of dielectric substrate material normally control variations in the dielectric constant to about 5%. Current measurement techniques do not have an accuracy sufficient to determine the dielectric constant with significantly greater accuracy. Consequently, uncontrollable variations in the dielectric constant between lots of substrate often result in large shifts in resonance frequency when the same antenna design is fabricated from new lots of substrate. Due to the already narrow bandwidth of the microstrip antennas, shifts in resonant frequency often severely degrade gain and performance of the antenna at desired frequencies. Therefore, a microstrip antenna is typically designed around a single lot of substrate material. Antennas fabricated in subsequent runs with different lots of substrate must sometimes be scrapped even when the dielectric constant is within manufacturer's tolerances for the original lot.
A microstrip antenna element can be tuned by attaching metal stubs to the metal layer forming the antenna element, or by cutting away tuning stubs formed with the metal layer. However, once a microstrip antenna element is assembled with a radome covering or with other microstrip antenna elements as a layer of a multi-layer microstrip antenna, the antenna can no longer be tuned.